In Gaul in 52 B.C., two Roman soldiers, Legionary Titus Pullo and Centurion Lucius Vorenus, are tasked with recovering Julius Caesar's personal Eagle, stolen from his camp in the dead of night. With his campaign in Gaul coming to a successful conclusion, Caesar's popularity is continuing to grow. He's saddened however when he receives news from his good friend Pompey Magnus that his daughter, Pompey's wife, has died in childbirth. In the Senate, Pompey must defend the prolonged absence of his friend and co-Consul Caesar against charges of corruption and of waging an illegal war. It's all a ruse however as he is plotting to eliminate him. Meanwhile, Atia of the Julii sends her son Octavian to Gaul deliver a gift of a beautiful stallion to his great uncle Julius Caesar. He is taken prisoner along the way. Fortunately, Vorenus and Pullo rescue him and as a result, both receive Caesar's favor.

Caesar winters his army in Gaul near the Italian border while his enemies in Rome fear that he will do something no Roman General has done: cross the Rubicon with his army. To counter-balance his opponents, Caesar puts his friend Mark Antony up for office as the people's Consul. Vorenus and Pullo accompany Mark Antony and Octavian to Rome. For Pullo, it's an opportunity to get drunk and ravish every woman in sight. He also loses all of his money in a crooked dice game leading to a major brawl. For Vorenus, he will have a reunion with his wife Niobe who he has not seen for 8 years. He's told that the baby he finds in his home is his grandson but Niobe is hiding a secret. In the Senate, there is a move to have Caesar declared a traitor but when Mark Antony tries to return to the Senate to veto their decision, Pullo is attacked by someone he fought the night before. Antony thinks the attack is against him and Pullo kills the man and Caesar declares Pullo a hero.

With Caesar quickly advancing toward Rome, Pompey begins ordering a retreat and forces his citizens to pick sides. Timon does Atia's dirty work and Vorenus cannot understand why no one is resisting while on a scouting mission.

Brutus gets a chilly reception from Servilia when he returns home from Greece. In Egypt, Caesar rebukes the advisers of the boy king, Ptolemy XIII, for their presumption in eliminating Pompey and demands the man who killed him. Caesar decides to intervene in the dispute between Ptolemy and his sister-wife, Cleopatra, to ensure both Rome's grain supply and his own access to Egypt's treasure. Antony, in an unusual show of prudence, advises against this with only half a legion in Alexandria and Cato and Scipio still at large in Africa. Caesar sends Vorenus and Pullo to rescue Cleopatra from house-arrest and escort her to Alexandria. Upon meeting Vorenus and Pullo, the narcissistic, opium-smoking teenage Cleopatra decides that her only hope of survival is to seduce Caesar and she compels Vorenus and Pullo to help smuggle her into the city first by unsuccessfully attempting to seduce Vorenus, but she has better luck with Pullo. Servilia's relationship with Octavia deepens beyond friendship into a passionate lesbian tryst. Caesar is besieged in Alexandra by the Egyptians loyal to Ptolemy. Back in Rome, Antony greets Brutus and Cicero on the floor of the deserted Senate and makes Cicero a promise to kill him if he ever turns against him.

The price of Caesar's mercy: Cicero nominates Caesar to an unprecedented ten-year dictatorship and Brutus speaks in support of the motion. With martial authority and Niobe at his side, Vorenus campaigns for magistrate under Posca's advice, but is shocked to learn from Posca that Caesar has already fixed the election in Vorenus's favor. Meanwhile, Atia continues her humiliation of Servilia while denying any involvement in her assault. But Atia becomes troubled by the silent depression of Octavia after Octavian compels her to return home from the cloister where she had fled. As Servilia lies at home shattered by her recent attack, she soon revives and gets her self-confidence back when she takes in the homeless and embittered Quintus Pompey whom she sees as her tool for revenge against Caeser for leaving her. As Caesar prepares for his Triumph involving five days of celebration and games, Antony finds that the dictator has no sense of humor about the trappings of power. The circulation of a denunciation of Caesar, signed by Brutus, fascinates Rome but stuns Brutus, who suspects his mother. Denied permission to march with the Thirteenth in the Triumph, the dejected Pullo resolves to make a new life as a civilian with Eirene, but his dream leads only to horror for the Vorenii and utter wretchedness for himself.

Vorenus' defense of Caesar lands him in an unexpected position of power within Rome. Meanwhile, Servilia hurls the final obstacle in her ambitious and complex revenge plan against Atia.